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Evidence for speciation without geographic isolation seen in “Evolution Canyon”

Habitat divergence in “Evolution Canyon”. (A) photo showing part of the “African” slope/south-facing slope of Evolution Canyon, characterized by light terra rossa soil with a stretch of grass cover that generates a yellow-brownish background. (B) Photo of part of the “European” slope/north-facing slope, characterized by dark terra rossa soil and shady, humus-laden dark background. doi:10.1371/journal.pone.0008708.g002

Habitat divergence in “Evolution Canyon”.
(A) photo showing part of the “African” slope/south-facing slope of Evolution Canyon, characterized by light terra rossa soil with a stretch of grass cover that generates a yellow-brownish background. (B) Photo of part of the “European” slope/north-facing slope, characterized by dark terra rossa soil and shady, humus-laden dark background.
doi:10.1371/journal.pone.0008708.g002

The birth of a new species, the event known as speciation, is usually thought to happen when one species gets split into two or more physically isolated populations that diverge over time as they accumulate differences without interbreeding. However, it might be possible in principle for sympatric speciation to occur — that is, for a new species to evolve within a freely interbreeding population without geographic isolation.

Now scientists find evidence of an incipient case of sympatric speciation occurring with mammals — spiny mice of the genus Acomys — in what they call “Evolution Canyon” in Israel. The findings are detailed in the Proceedings of the National Academy of Sciences.

The existence of sympatric speciation as a mechanism for the origin of species has been hotly contested since it was first proposed by Darwin in 1859. For instance, there is still skepticism that sympatric speciation is extensive in nature, except polyploidy in plants, says evolutionary biologist Eviatar Nevo at the University of Haifa in Israel.

“The controversy on sympatric speciation is still ongoing and is a central issue in evolutionary biology,” Nevo says.

Nevo and his colleagues recently argued that blind subterranean mole rats, Spalax galili, might be experiencing sympatric speciation in the eastern Upper Galilee Mountains in northern Israel. They also speculated that sympatric speciation might be abundant in nature.

Now the scientists describe a similar case of sympatric speciation elsewhere in northern Israel at Mount Carmel, in a site they came to call “Evolution Canyon” and the “Israeli Galapagos” due to incipient sympatric speciation they have also seen there in bacteria, fungi, plants such as wild barley, and insects such as Drosophila and beetles.

“I started to explore ‘Evolution Canyon’ in 1990,” Nevo says. “We identified, in an area of 7,000 square meters, some 2,500 species across life and studied 16 model organisms in depth. The model organisms include soil bacteria, fungi, flowering plants and all animals from worms to insects, lizards, and mammals.”

The opposite slopes at “Evolution Canyon” vary dramatically from each other. The south-facing slope receives a lot of sunlight and is thus dry and hot, reflecting an African savannah-like environment, while the north-facing slope just 200 meters away on average is relatively humid, cool and forested, resembling a southern European setting.

The researchers investigated spiny mice of the genus Acomys, which is composed of about 19 tropical rodent species in rocky habitats in Africa and southwest Asia. They focused on Acomys cahirinus in “Evolution Canyon,” which fossil evidence suggests colonized Israel about 30,000 years ago.

The scientists discovered significantly more diverse nuclear and mitochondrial DNA in Acomys on the canyon’s Africa-like slope than the Europe-like slope. The mice also differed significantly physically in terms of shape, physiology and behavior. For instance, spiny mice on the Africa-like slope eat more snails, perhaps because such prey is richer in water and thus more desirable on that hotter, drier slope; the rodents on the Africa-like slope also consume 20 percent less oxygen, probably to conserve energy in the more hostile environment. Preliminary tests suggested the mating choices these spiny mice took also depended on which slope prospective mates came from.

The researchers suggest Acomys first colonized the Africa-like slope of the canyon, with some then moving to the Europe-like slope about 20,000 years ago. The sharp ecological differences between the populations may one day cause them to become distinct species.

“Basically, we substantiated Darwin’s hypothesis that sympatric speciation is one of the realistic models of the origin of new species in nature,” Nevo says.

Categories: Evolution
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